This invention relates generally to on screen display systems for television receivers and specifically to on screen display systems for developing subtitles on the television receiver viewing screen for the benefit of the hearing impaired. More specifically, this invention is concerned with a method of reliably identifying the particular line in the vertical blanking interval (VBI) of the received television signal.
Modern television receivers often incorporate on screen displays, usually for displaying the time, date and channel number. Such displays are also used for controlling various functions such as volume, color, tint, etc. and for so-called picture-in-picture displays. The size and location of the displayed information is often viewer-controllable by means of a remote control device and the microprocessor in the television receiver. Many television receivers also include "teletext" through which a wide range of print and graphics information may be presented to a viewer. Teletext information is received from coded data that is transmitted in the VBI of the television signal, specifically on one or more designated horizontal lines in the VBI.
Recently, in an effort to better serve viewers who are hearing impaired, the FCC has mandated that certain classes of television receivers must be capable of displaying explanatory subtitles with the video picture. The subtitles will enable a hearing impaired viewer to follow the sense of the dialogue accompanying the televised program and will be available when the receiver is placed in a caption mode by operation of a caption display control. In the normal television viewing mode, the caption feature will not be activated and subtitles will not appear on the television screen.
The captioned data will be encoded on line 21 of the odd field of the television signal and must have specific characteristics. The information on line 21 will consist of a run-in signal, comprising seven cycles of a 0.503 megahertz clock signal immediately following the end of the horizontal sync signal blanking level. The run-in signal is followed by a start sequence consisting of two 0 bits and a 1 bit, which is referred to as the start bit. The start bit is in turn followed by two bytes of either control information or data. The run-in signal commences at 10.074 microseconds from the leading edge of the line 21 horizontal sync pulse, the start bit commences at 27.452 microseconds from the start of horizontal sync and the following two bytes of data (or control information) occupy another 33.764 microseconds. The captioned data only appears on line 21 and is not present in the corresponding line in the even field. Consequently, the system for recovering the captioned data must be able to find line 21 to recover that data.
Conventional line detection circuits are well known in the art for detecting data on selected lines in the television signal VBI. The format of NTSC television signals is such that the line numbers commence after the vertical sync pulse. In a receiver where the horizontal and vertical synchronizing signals are developed by countdown circuits, any particular line number may be readily found. In analog receivers and in VCRs, the nature of vertical sync recovery circuits precludes the use of a simple horizontal pulse counting arrangement to locate a given horizontal line. With such analog recovery circuits, it is not uncommon to reach line 21 by counting 16 or 17 horizontal pulses after the recovered vertical sync pulse. Further, in VCRs, artificial vertical blanking intervals are generated during special effects such as pause and slow motion and one cannot simply count horizontal sync pulses (after recovery of the vertical sync pulse) to determine a particular line in the VBI.
As mentioned, where the horizontal and vertical timing signals are derived by countdown circuits from a common clock, the problem of identifying horizontal line 21 is relatively simple. However, clock countdown circuits are not used in the majority of analog television receivers and the need to quickly and economically bring closed captioning to the marketplace has presented television manufacturers with a major obstacle. Thus there is a need in the art for a low cost closed caption detection system that 1) can reliably find the VBI line that carries the closed captioned data, 2) adjusts the "slice" level of the data for reliable detection, and 3) samples the data reliably without elaborate synchronizing systems.